In manufacturing a moving web, such as a tissue sheet, it is desirable to produce a basesheet that is soft. Similarly, it is desirable to manufacture the web at as high a rate possible to minimize manufacturing cost. Unfortunately, a moving web such as a tissue sheet becomes unstable when transported at high rates of speed unless undue tension is applied to the web. For a low modulus tissue web it is not possible to apply a level of tension to completely stabilize the high-speed moving web without pulling out the crepe reducing softness. Therefore, there is a desire and need by manufacturers to improve the methods of transporting a low tension web, such as a tissue web, at a high rate of speed while still maintaining sheet stability and avoiding product damage.
It has now been discovered that high speed tissue webs can be stabilized within the dry end of a tissue machine by the proper placement and use of two or more sheet handling elements, namely a creping blade foil, an airfoil and a roll foil. Stabilization of the web can permit the speed of the paper machine to be increased. Alternatively, the associated waste and delay of the paper machine can be reduced if the machine is already speed limited. While the entire system is directed towards stabling a tissue web in a paper machine, the system""s elements can be used to stabilize any moving web in any web processing machine.
As used herein, an xe2x80x9cairfoilxe2x80x9d is a substantially web wide sheet handling element intended to stabilize a moving web. An airfoil may be active in that it might use compressed air or vacuum to enhance or augment the airfoil""s natural ability to stabilize the moving web. An airfoil also may be passive and rely solely on the web""s movement to attract the web to the airfoil. A few examples of airfoils, and by no means exclusive, include a simple flat plate, a common oval shaped airfoil having two flat parallel surfaces with rounded leading and trailing ends, and the airfoil shown in FIG. 3.
A xe2x80x9cfirst locationxe2x80x9d means any point in space the moving web occupies prior to a rotating roll in a web handling process.
A xe2x80x9csecond locationxe2x80x9d means any point in space the moving web occupies after being creped off the surface of a rotating roll or a Yankee dryer.
A xe2x80x9crotating rollxe2x80x9d means a circular roll, rotating about its axis and includes, but is not limited to, a reel drum, a winder drum, a Yankee dryer, a guide roll, and an idler roll.
Hence in one aspect, the invention resides in a method for transferring a moving web from a first location to a rotating roll comprising: (a) stabilizing the moving web on an airfoil located between the first location and a roll foil; (b) directing the moving web toward the roll foil having an internal volume, a back surface, and a gap in the back surface in fluid communication with the internal volume and connected to a source of vacuum; the placement of the roll foil defining a region bounded by the rotating roll on a first side, the back surface on a second side, and the web on a third side; and (c) extracting from the region through the gap at least a portion of the boundary layer air traveling with the rotating roll.
In another aspect, the invention resides in a method for transferring a moving web from a creping blade in a creping blade holder to a second location comprising: (a) stabilizing the moving web on a creping blade foil located beneath the moving web having a top surface with a front edge and a back edge, the front edge touching or in close proximity to either the creping blade or the creping blade holder; (b) directing the moving web towards one or more airfoils located between the creping blade foil and the second location, at least one airfoil having a bottom, a top, a nose, and a tail, the bottom being essentially flat, the nose being a curved surface attached to the bottom and the top, the top being another curved surface extending from the nose to the tail meeting with the bottom at an edge; and (c) stabilizing the moving web on the airfoils.
In another aspect, the invention resides in a method for transferring a moving web from a creping blade in a creping blade holder to a rotating roll comprising: (a) stabilizing the moving web on a creping blade foil having a top surface, a front edge, and a back edge, the creping blade foil being located beneath the moving web with the front edge touching or in close proximity to the creping blade or the creping blade holder; (b) stabilizing the moving web on at least one airfoil located between the creping blade foil and a roll foil, the roll foil having an internal volume, a back surface, and a gap in the back surface in fluid communication with the internal volume connected to a source of vacuum; the placement of the roll foil defining a region bounded by the rotating roll on a first side, the back surface on a second side, and the web on a third side; and (c) extracting from the region into the internal volume of the roll foil by vacuum through the gap at least a portion of the boundary layer air traveling with the rotating roll whereby the moving web is transferred to the rotating roll with minimal disturbance.
In another aspect, the invention resides in an apparatus for transferring a moving web to a rotating roll comprising: (a) a roll foil having an internal volume, a front surface, a back surface, and a top surface, the placement of the roll foil defining a region bounded by the rotating roll on a first side, the back surface on a second side, and the web on a third side; (b) a boundary layer reduction member attached to the roll foil adapted to reducing the boundary layer of air traveling with the rotating roll; and (c) a gap in the back surface in fluid communication with the internal volume of the roll foil connected to a source of vacuum.
In yet an additional aspect, the invention resides in an apparatus for transferring a moving web to a rotating roll comprising: (a) a roll foil located adjacent a rotating roll having an internal volume, a front surface, and a back surface; (b) a gap in the back surface in fluid communication with the internal volume and connected to a source of vacuum adapted to removing at least a portion of the boundary layer of air traveling with the rotating roll; and (c) a boundary layer reduction member attached to the back surface.
In one preferred embodiment, a creping blade foil having a front edge, a back edge, and a top surface is placed immediately following the creping blade touching the creping blade holder. The tissue web, after creping, then runs adjacent to the creping blade foil""s top surface and leaves the foil parallel to the top surface at the foil""s back edge. The creping blade foil stabilizes the tissue web, after the instability of the creping process, and directs the web to a second location. Between the second location and the creping blade foil, at least one other airfoil is placed adjacent to the web.
In another preferred embodiment, a roll foil is provided to smoothly transfer the moving web to a rotating roll, such as a reel drum. The roll foil includes a top surface, a back surface, a front surface, and has an internal volume connected to a source of vacuum. In operation, the roll foil is placed near the rotating roll. Preferably, the back surface is arcuate and matches the radius of the rotating roll such that the roll foil can be placed immediately adjacent the rotating roll to minimize the distance between the roll foil and the roll""s surface. The roll foil is placed such that the web runs adjacent to the top surface, and then the web is transferred to the rotating roll""s surface. The transfer of the web in this manner defines a region or pocket bounded on one side by the rotating roll, on a second side by the roll foil and on a third side by the tissue web. A gap, located in the back surface of the roll foil, is in fluid communication with the roll foil""s internal volume. A source of vacuum then draws air though the gap into the roll foil. This eliminates at least a portion of the boundary layer air traveling with the rotating roll, promoting a smooth transfer of the web to the rotating roll""s surface. Alternatively, the vacuum can be adjusted to provide a negative air pressure in the region pulling the tissue web onto the rotating roll""s surface.
Without a roll foil the tissue web tends to billow away from the rotating roll""s surface due to boundary layer air traveling with the surface of the rotating roll. If the roll is a reel drum wrinkles can be wound into the product from the billowing. Furthermore, such billowing severely reduces the efficiency of the automatic turn-up sequence for a paper machine reel. The roll foil also serves to reduce machine-direction tension variations of the moving tissue web. Web stability is improved if the web tension is constant and not varying thereby reducing web flutter upstream of the roll foil.
In order to eliminate some of the boundary layer air traveling with the rotating roll a boundary layer reduction member, such as a wiper, is attached to the roll foil""s back surface and is in contact or close proximity to the rotating roll""s surface. The wiper has been found an integral part of the system because it prevents the accumulation of dust and debris from entering the space between the roll foil and the rotating roll. Furthermore, the wiper reduces the amount of airflow required though the gap to prevent billowing by initially blocking a portion of the boundary layer air traveling with the rotating roll. The boundary layer reduction member could be a simple rubber flap or a more complicated doctor blade.
An alternative boundary layer reduction member is an air shower. This is desirable for applications involving a rough coated roll surface such as plasma coating. It is not as practical to use a wiper or doctor blade as a boundary layer reduction member on such a surface since the contact could wear the surface and the wiper. The air shower works by creating a low-pressure area upstream of the airflow entraining additional air. Because the air shower is directed to oppose the rotation of the rotating roll, it strips off part of the boundary layer air traveling with the rotating roll, and entrains upstream air from the region helping reduce the air pressure in the region. The air shower is also useful to clean the rotating roll""s surface, and to prevent debris during a web break from entering the region or the gap.
In operation, a moving web located at a first location traverses a span to the roll foil and is transferred to the rotating roll. The roll foil eliminates at least a portion of the boundary layer air traveling with the rotating roll reducing or eliminating billowing of the web at the point of transfer to the roll. Between the first location and the roll foil, at least one other airfoil is located to stabilize and direct the web to run adjacent to the roll foil""s top surface.
In the most preferred embodiment, all three sheet handling elements are utilized. The moving web is initially stabilized by the creping blade foil, further stabilized and directed by a plurality of airfoils, and then finally transferred to the rotating reel drum""s surface by the roll foil. It has been found that only using the creping blade foil and at least one other airfoil to stabilize the initial portion of the web run enhances sheet handling. Similarly, using only the roll foil and at least one other airfoil is effective to stabilize only the final portion of the web run. These alternatives are considered part of the claimed invention, along with use of all three sheet handling elements.